The synthesis of Beta h-EP-(1-15),-(1-21),-(1-26),-(1-28),-(1-29), and -(1-30) have been accomplished by the improved procedure of the solid phase method. Opiate activity of these analogs was assayed by the guinea-pig ileum procedure. Results show that extension of the met-enkephalin at its COOH-terminus to give Beta-EP-(1-21) brings back activity exceeding that of the NH2-terminal pentapeptide. No significant increment of activity is observed by further addition of five more residues to form Beta-EP-(1-26). The most dramatic gain in activity occurs as the chain is lengthened by addition of the next 3 residues. Thus, Beta h-EP-(1-29) displays the full activity of Beta h-EP in the guinea pig ileum assay. The analgesic activity of synthetic peptides was assessed in mice by the tail-flick method. Beta h-EP caused a dose-dependent inhibition in intensity and duration of tail-flick response when administered in doses of 0.05-0.43 micron g per mouse. The AD50 of Beta h-EP was calculated to be 0.15 per mouse. Results show that the removal of one amino acid at a time starting with the COOH-terminus of Beta h-EP reduced the potency of analgesic activity of the Beta h-EP in a stepwise fashion. Thus, removal of COOH-terminus Glu reduced analgesic activity by 28%, removal of -Gly-Glu reduced analgesic activity by 80%, and removal of -Lys-Gly-Glu reduced analgesic activity by 94%. Thus, complete primary structure is required for full analgesic activity of Beta h-endorphin. Tritiated human Beta-EP, specifically labeled at Tyr-27 has been prepared with 50 Ci/mmole. (3,5-I2-Tyr27)-Beta-EP was synthesized by the solid phase method. The iodinated peptide (6 mg) was reduced catalytically with tritium gas. After purification, it yielded 1.4mg tritiated Beta-EP with 50 Ci/mmole. The biological activity of tritiated peptide was identical to the native Beta-EP as assayed by the guinea-pig ileum procedure.